1. Field of the Invention
The present invention relates to a power supply unit. The power supply unit according to the present invention is used as a power supply input circuit to be connected to the front stage of a switching power supply or used as a switching power supply.
2. Discussion of Background
Power supply units of this type in the known art are represented by the boost type and the buck-boost type. A typical boost type power supply unit is provided with a basic circuit structure that includes a rectifying circuit and a smoothing capacitor with a choke coil connected between them. A switching element is connected between power lines that are on a rear stage of the choke coil and a diode is connected in series with a power supply line that is on a rear stage of the switching element. Furthermore, an output capacitor is connected between the power lines on the rear stage of the diode and both ends of the output capacitor are led to an output terminal. In this circuit structure, the output capacitor is charged in one direction with the energy supplied by the power source and the energy stored in the choke coil while the switching element is ON via the diode to generate boosted voltages at both ends of the output capacitor.
A typical example of the buck-boost type power supply unit is a flyback converter system in which a switching element is connected in series with the input winding of the transformer for conversion and the energy stored in the transformer for conversion while the switching element is ON is transferred to the load side during the next OFF period.
Japanese Unexamined Patent Publication (KOKAI)No. 307365/1990 discloses an example of the buck-boost type power supply unit. The power supply unit disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 307365/1990 is provided with a full wave rectifying circuit and a capacitor that is connected between the rectified output terminals of the rectifying circuit, with another switching element provided between the rectifying circuit and the inductor so that the two switching elements are turned ON and OFF at approximately the same time to obtain either a boosted or a stepped-down output voltage. However, the power supply units in the known art described above present the following problems.
First, in a boost type power supply unit, there is only a switching element connected between the power lines and there is no switching element to be connected in series to the power lines. Because of this, when the switching element has stopped its switching operation and is in an OFF state, the input voltage is output through the lines without regulation. Furthermore, if the output terminals are shorted or overloaded, protection against over current cannot be implemented. In addition, when the input voltage is applied, a rush current runs to the smoothing capacitor, and it may cause electrical damage to the rectifying circuit.
Another problem is that, since the output voltage is always higher than the input voltage, a DC-DC converter with high allowable input voltage is required on the rear stage. As for a buck-boost type power supply unit, since it is provided with a transformer for conversion which isolates the input terminals from the output terminals, unlike a boost type power supply unit, it does not present problems caused by the absence of a switching element connected in series to the power supply line. However, in this case, the entire energy must be transferred via the transformer for conversion. This, in turn, leads to lowered efficiency in energy conversion and a larger circuit.
The power supply unit disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 307365/1990 requires that complicated ON/OFF control be performed for its two switching elements, making control difficult. Also, since the two switching elements and the two diodes are connected to the choke coil in series, the power loss is great when energy is stored in the choke coil and when the stored energy in the choke coil is discharged. This reduces the efficiency.